1. Field of the Invention
This invention relates to computer systems and, more particularly, to heat sinks used to dissipate heat in computer systems.
2. Description of the Related Art
Electronic enclosures may contain many different electronic devices that are designed to operate within certain temperature ranges. However, when operating, each electronic device may generate heat. If enough heat is generated within the enclosure to cause any of the enclosed electronic devices to operate outside of their operational temperature range, problems may occur. For example, in some cases, increased temperature may cause a system to malfunction or behave erroneously. Sometimes, increased heat may even damage the electronic devices or components within the enclosure.
In order to reduce heat-related problems, many electronic enclosures include cooling devices. One simple example of a cooling device is a passive heat sink that radiates heat from a device into the surrounding air. A passive heat sink is simply a piece of metal attached to a component in such a way that the component transfers heat to the heat sink. By increasing the surface area from which heat can be radiated, heat sinks increase the amount of heat that can be transferred from a component to the surrounding air. Some heat sinks have fins, which further increase the surface area and allow even more heat to be radiated away. In some systems, passive heat sinks may be all that is needed to ensure proper cooling. For example, convection will cause heated air to rise higher than cooler air, so in some cases, hotter air will naturally be circulated away from the heat-generating component while cooler air is constantly being circulated towards the component. In other systems though, factors such as the size of the enclosure or the orientation of the device within the enclosure may limit the beneficial effects of convection. In such situations, other cooling devices may be needed to prevent heat-related problems.
One problem that arises in systems that simply radiate heat is that the heat may accumulate inside an area in the enclosure. For example, convection may no longer assist in cooling if all of the air in an area becomes equally heated. One situation where this might arise is if a heat-generating component is located near the top of an enclosure. The top component may quickly lose any benefits of convection due to the accumulation of heated air in the top of the enclosure. Another situation where this might arise occurs when many devices are mounted in close proximity in an enclosure. Each device""s ability to radiate heat away from itself may be limited if the surrounding air has already been heated by the other components.
In order to alleviate problems that arise when heat accumulates, many systems incorporate devices that can move already-heated air away from an area and draw less-heated air into the area. Air movers such as fans and blowers (e.g., centrifugal fans) are popular cooling devices because they are capable of moving heated air away from and/or cooler air towards areas where heat-related problems may arise. By regularly moving heated air away from or cooler air over a component, the component""s ability to radiate heat is better maintained. For example, air movers may move either warmer or cooler air to another section of an enclosure, move heated air from inside an enclosure to the outside of the enclosure, or move cooler air from outside an enclosure to the inside of the enclosure. Often, air movers are mounted onto passive heat sinks to create active heat sinks.
As processing speeds increase and die sizes decrease, electronic devices are decreasing in size and generating increasing amounts of heat. Consequentially, heat sinks are required to dissipate increasing amounts of heat away from smaller areas. One way to improve heat sink performance is to increase the size and/or mass of the heat sink. This in turn often increases the weight of the heat sink. Increased heat sink weight may increase the risk that the heat sink may decouple from and/or damage the device it is attached to if the chassis containing the heat sink is dropped or bumped. In order to reduce this risk, the heat sink may be coupled to the device it is cooling by a force that is a function of the weight of the heat sink. Often, this force is supplied by spring clips.
Heat sink attachment may be further complicated by the form factor of the device to be cooled. For example, the micro PGA form factor requires a surface-mount socket due to routing constraints. With a surface-mount socket, the semiconductor package cannot be allowed to move in an axis parallel to the package pins. As a result, forces much larger than have previously been necessary for heat sink attachment are now required for adequate heat sink attachment. Additionally, the printed circuit board to which the socket is mounted cannot be significantly deformed by the heat sink attachment force or by any force imparted to the heat sink assembly during a mechanical shock. Furthermore, during the assembly process, the forces applied to the surface mount socket need to be applied uniformly to prevent solder damage (e.g., solder crazing).
Currently, heat sinks are attached by fastening two plastic retainers to the printed circuit board, threading a spring clip into a heat sink and fan assembly, placing the heat sink and fan assembly with the spring clip onto the plastic retainers, aligning the heat sink and fan assembly over the processor, snapping an end of the spring clip into one plastic retainer, and using downward force to snap the other end of the spring clip into the remaining plastic retainer. A force equal to the spring force of the spring clip may need to be supplied in order to initially couple the heat sink to the device it is to cool. This force is usually supplied by assembly line workers. As heat sink weights increase, increasing amounts of force are required to attach the heat sinks. This leads to increased strain on workers.
Various embodiments of an improved heat sink subassembly are disclosed. In one embodiment, a heat sink subassembly may include a retainer comprising several attachment points, a heat sink coupled to the retainer, and a force-generating device. The heat sink includes several fins, one of which is shorter than the others. The force-generating device is coupled to at least one of the attachment points and to the shorter fin. The force-generating device is configured to exert a force that keeps the heat sink securely coupled to the retainer when the force-generating device is coupled to the attachment points. In some embodiments, the heat sink subassembly may include an electromagnetic shield coupled to the retainer (e.g., by a retaining ridge extending along an inside surface of the retainer) that may be inserted into and removed from the retainer without decoupling the heat sink from the retainer.
In another embodiment, a computer system may include a printed circuit board, an integrated circuit coupled to the printed circuit board, and a heat sink subassembly coupled to the printed circuit board. The heat sink subassembly may include a retainer, a heat sink that includes several fins, one of which is shorter than the others, and is coupled to the retainer, and a force-generating device coupled to the shorter fin. The force-generating device is configured to exert a force on the heat sink. The force keeps the heat sink securely coupled to the integrated circuit.
In other embodiments, a computer system may include a printed circuit board that includes several holes, an integrated circuit, and a heat sink subassembly. The heat sink subassembly may include a retainer coupled to the printed circuit board by no more than two fasteners, where each of the fasteners extends through a respective one of the holes in the printed circuit board, a heat sink coupled to the retainer, and one or more force-generating devices coupled to the heat sink and the retainer. The one or more force-generating devices exert a force on the heat sink that securely couples the heat sink to the integrated circuit. An installation force required to couple the heat sink subassembly to the printed circuit board and to cause the one or more force-generating devices to exert the force on the heat sink is less than the force exerted on the heat sink by the one or more force-generating devices.
A method of assembling a computer system may involve attaching a force-generating device to a retainer to form a heat sink subassembly in which a heat sink is interposed between a portion of the force-generating device and a portion of the retainer and where the attaching causes the force-generating device to hold the heat sink in the retainer; placing the heat sink subassembly over an integrated circuit coupled to a printed circuit board; and coupling the heat sink subassembly to the printed circuit board by exerting an installation force, where the installation force is less than a force exerted by the force-generating device on the heat sink as a result of said coupling.